The invention relates in general to electronics technology in wireless communications and in particular to the control of transmission power as specified in the preamble of the first claim.
Optimization of transmission power is important in a portable terminal, for example, because if the transmission power is too low, it may cause annoying breaks in the connection or the connection may be lost altogether. If, on the other hand, the transmission power is too high, it may interfere with nearby devices and unnecessarily drain the power source of the portable device.
Electronic components are known in which the resistance depends on the voltage between input and output. There are also components in which the resistance between the input and output can be varied by means of a control signal additionally coupled to the component. Such components will be hereinafter called voltage variable resistors (VVR). Typically, VVRs are used e.g. in voltage variable attenuators (VVA) employed in radio device transmitters.
FIG. 1 shows a diagram illustrating the use of a VVA in a RF transmitter with a typical direct-conversion-type transmitter arrangement. In the arrangement there is a modulator MOD 106 with input pins I 108 and Q 109 for incoming signals, an oscillator LO 107, amplifier MPA 105, voltage variable attenuator VVA 104, a second amplifier PA 103, band-pass filter DUPLEXER 102, shown with a receive branch output Rx 102B, and an antenna ANT 101.
The arrangement of FIG. 1 includes a VVA 104 to control the transmission power of the TX chain depicted. Linearity of the transmitter VVA dictates the level of amplification needed in the TX chain following the VVA. There is also a relationship between linearity and power handling capacity such that more linear VVAs can handle more power. Therefore, less post-VVA amplification is needed. Post-VVA amplification, in turn, influences the broadband noise minimum level at the transmitter, which is significant regarding, among other things, the output power dynamic range at the transmitter and TX-RX isolation requirements in the duplex filter.
A VVR can be implemented using semiconductors, for example. A conventional FET can be set to its operating point in such a manner that the drain-source resistance can be varied by the gate voltage whereby the FET can be used as VVR.
FIG. 2 depicts a prior-art single-chip FET-based circuit set up as a VVA 104. The VVA has an input (IN) 201 and an output (OUT) 206 and a RF line where the components are RF line elements 203, 204, and 205. The FET 210 is connected to the junction point of RF line elements 203 and 204 by its drain electrode, and FET 208 is connected to the junction point of RF line elements 204 and 205 by its drain electrode. A first end of resistor 207 is connected to the gate of FET 208. A first end of resistor 211 is connected to the gate of FET 210. Second ends of resistors 207 and 211 are connected to a control input (V1) 202 to produce gate control. The source electrodes of FETs 208 and 210 are connected to the ground potential via points 209B and 209A respectively.
The control voltage V1 of the VVA gate control 202 in FIG. 2 is used to control the drain-source resistance of the FETs functioning as VVRs, which controls the attenuation of the whole VVA, i.e. the ratio L of the input and output power. Resistors 211 and 207 are used to increase isolation between the RF line (elements 203, 204 and 205) and the gate control 202.
FIG. 3A illustrates the interdependence of input power PIN and output power POUT in a VVA. The interdependence is in accordance with the straight line 301 in an ideal case, but in practice the interdependence is not linear but may be in accordance with the curve 302. FIG. 3A shows points 303 and 304 for the input power axis PIN 305 and output power axis POUT 306 where the deviation equals one decibel milliwats referred to one decibel milliwatt, −1 dBm.
One measure of attenuator linearity is the −1 dB compression point. One way of characterizing the attenuator and especially its linearity is to express the −1 dB attenuation compression point dependence as a function of the attenuation ratio L.
To provide an example, FIG. 3B shows a curve 307 describing said dependence of an attenuator according to the prior art. The technical specifications of the system are selected according to the lowest linearity point of the attenuator, which in FIG. 3B is the minimum of curve 307, located at point 310 of the compression point axis 308, and at point 311 of the attenuation ratio axis 309. To improve the linearity it is possible to attempt to raise the minimum in the direction where the values of the compression point axis 310 get higher.
It is possible to attempt to solve the linearity problem by connecting FETs in series or using smaller FETs, but in both of these cases the minimum resistance of the VVR increases, which in practice will narrow down the dynamic range and usually causes additional losses. The linearity of a FET's drain-source resistance varies considerably as a function of the gate voltage. The linearity gets worse when adjusting the FET from the minimum resistance state towards higher resistance values whereby nonlinearity grows detrimental.
VVRs fabricated using typical monolithic GaAs processes are not linear enough, which in practice hampers their use in the VVAs of radio devices. It is known to make MESFETs (Metal Semiconductor Field Effect Transistors) and PHEMTs (Pseudomorphic High Electron Mobility Transistors) and VVRs that can be implemented using single FETs with a variable resistance and/or impedance. Their drawback is poor linearity. Therefore, the linearity of the VVA operation when implemented with a typical FET-based VVR, is rather poor as well.
Alpha Industries Inc. is known to have developed a process for fabricating triple-gate 0.25-μm MESFETs which are used e.g. in VVR applications as VVA elements (Alpha part #AV850M2-00). Linearity for said attenuator is fairly reasonable, the input compression point is in the order of +10 dBm in all attenuation states.